Precipitation method for the separation of plutonium and rare earths



RREtmmtfloN METHOD non THE SEPARATION or PLUTONIUM AND an EARTHS StanleyThompsom Richmoud, Calif-., assignor to the United States of America asrepresented by the United States Atomic Energy Commission No Drawing.Application November 17, 1948 Serial No. 60,617 v 2 Claims. (Cl, 23-145)Ibis invention relates to the separation and purification of plutoniumand particularly to the separation of plutonium from radioactiveelements of lower atomic weight. This invention is also concerned with anew and useful composition of plutonium.

In this specification and claims, the name of the element .designatesgenerically the element both in its free state and combined in acompound. The element in its free state is designated by the termelemental or by its specific state, such as fmetallici An object of thepersent invention is to provide a method for separating plutonium fromcertain elements with which it is contaminated as aresult ofitsformation by the neutron irradiation of uranium and particularly theseparation from the rare earths with which it is contaminated.

,A further object of the present invention is to provide anovelanduseful composition of plutonium. .1 Additional objects and advantages ofthis invention will be evident from the following description. 1

.In accordance with the present invention, it has been found thatplutonium ions in the tetravalent state may be separated from an aqueoussolution by precipitating the tetravalent plutonium ions withpyrophosphate anions. It has further been found that tetravalentplutonium ions contained :in an aqueous solution in a concentration solow that said plutonium ions will not beprecipitated directly as thepyrophosphate may be removed substantially quantitatively from thesolution with a carrier precipitate of .a'heavy ,metal'pyrophosphatewhich is insolublein an aqueousacidic solution. The pyrophosphates whichhave been found to be most useful as plutonium carriers include uranous,ceric-, zirconium-, and particularly thopyrpphosphate. These carrierpyrophosphates, for ex mple thorium pyrophosphate, have been found tocarry tetravalent plutonium ion "quantitatively, but thesepyrophosphates do not carry-the trivalent or hexavalent plutonium ion,:thetrivalent rare earth ions ortheuranyl ion.

tion to the ,hexavalent state, and re-precipitating the,

thorium pyrophosphategleaving the hexavalent plutonium ions in thesolution in a substantially pure state.

This process is not only useful in obtaining plutonium in anuncontaminated state, but is also useful inlobtainins radioactivefissionproducts. The usefulness of these appreciable extent.

These discoveries formthe basis of a process for septetravalent state.

Patented Apr. 26, 1960 tain acids are excluded depending upon the choiceof cation for the carrier precipitate. Thus, if thorium pyrophosphatewere to be used as a carrier, an-acid would not be chosen which wouldform an insoluble compound "with thorium. Other considerations, such asthe formation of complex ions, may exclude other acids in certaininstances. However, nitric acid and hydrochloric acid are highlysatisfactory in all cases and it is unnecessary to attempt to use otheracids for the adjustment of the pH of the solution. After the plutoniumis brought into the solutio'nthe H+ ion concentration of the solutionshould be adjusted so that the solution is acid. It is essential thatthe solution be acid prior to the precipitation step if the plutonium isto be separated from trivalent rare earth contaminants by theprecipitation step since the trivalent rare earth pyrophosphates mayprecipitate in other than an acid solution. The upper limit of the acidconcentration in the solution, however, is not critical since thecarrier and tetravalent plutonium pyrophosphates are not soluble even inconcentrated acids. For example, it has eenfound that a mixture ofthorium and plutonium pyrophosphate will not dissolve in '16 N nitricacid to any An acidity of between 0.5 and 2 N has been found to givesatisfactory results.

It is important that the plutonium be present in the solution in thetetravalent state since it has been found that only the tetravalentplutonium ions form an insoluble pyrophosphate in acid solutions.Relatively stable states of plutonium in the ionic state in aqueoussolution are the +3, +4 and the +6 states. The normal state of plutoniumwhen it is placed in solution by the dissolving of neutron irradiateduranium in nitric acid or hydrochloric acid is the +4 state, so that ifthe solution is formed in this manner, ordinarily, no furtherprecautions need be taken to insure that the plutonium is in solution inthe However, should the plutonium be placed in solution by any othermethod which would make the valence state of the plutonium ionsquestionable, the plutoniummay be treated with suitable oxidizing orreducing agents, as the case may be, to insure that it is in thetetravalent state. The oxidation-reduction potentials (referred to thenormal hydrogen electrode) linking Puf, Pu+ ,.and Pu+ in 1 M nitric acidat 30 C. are:

+0.92 Volt -l.1l Volts Pu+ Pu: Pu

1.0a volts solution with a carrier may be effected by the precipitationof the carrier in the solution, by the treatment of thesolution-containing plutonium with the preformed carrier, or by passingthe solution through a bed of the carrier. However, it has been foundpreferable to form the carrier in the solution. This may be done byintroducing a soluble compound containing the cation, for

example thorium chloride or thorium nitrate where thorium pyrophosphateis to be the carrier, into the solution. The carrier precipitate maythen be formed by introducing a soluble pyrophosphate compound, such aspotassium or sodium pyrophosphate orpyrophosphoric acid into thesolution. The order or method of contacting the plutonium-containingsolution with the carrier is not essential and variations may beintroduced without departing from the method of the invention. Thepyrophosphate carriers herein described are soluble in an excess ofpyrophosphate, presumably by the formation of pyrophosphate complexes,such as throium pyrophosphate complex. Although a precipitate of thoriumpyrophosphate can be formed where the molarity of the pyrophosphate insolution is ten times as great as the molarity of the thorium, it isdesirable to form the carrier pyrophosphate in a solution where themolarities of the cation and of the pyrophosphate do not differ toogreatly from stoichiometric proportions. It has also been founddesirable to carry out the carrier precipitation at room temperaturebecause of the tendency of tetravalent plutonium ions to oxidize in warmnitric acid, and the' precipitation is usually complete with relativelyshort periods of digestion even at room temperature. The thoriumpyrophosphate-plutonium pyrophosphate precipitate is a white crystallineprecipitate and it may be separated from the solution by filtration,decantation, or preferably centrifugation.

This step of precipitating tetravalent plutonium with a pyrophosphatecarrier may be used to separate plutonium from most elements with whichit is contaminated, except those which are normally tetravalent inaqueous acidic solution. Thus, this step will separate plutonium fromthe rare earths (including cerium, which will normally be present inaqueous solution in the trivalent state), Rb, Sr, Y, Cs, Ba, Kr, and Xe.The carrier precipitation step described above may also be used toseparate plutonium from uranium because by the normal process ofdissolving neutron irradiated uranium in nitric acid, the uranium ispresent in the uranyl nitrate hexahydrate thus formed in the hexavalentstate which is not carried with a pyrophosphate carrier, and theplutonium is present in the tetravalent state. This step of the processmay also be used as an analytical procedure to determine the relativeamounts of trivalent, tetravalent and hexavalent plutonium in solution,since the tetravalent plutonium will be carried with a pyrophosphateprecipitate, while the trivalent and hexavalent plutonium will remain insolution. The composition of ThPzOq-PUPzOq also has uses independent ofits uses outlined above. For example, plutonium may be convenientlytransported in the form of this composition.

The foregoing step, although useful individually for the purposesoutlined above, may also be employed as a step in the followingprocedure which is a more complete plutonium separation process. Thecarrier precipitate containing the plutonium pyrophosphate obtained inthe preceding step may be dissolved by treatment of the precipitate withan excess of pyrophosphate ion. For example, the thorium-plutoniumpyrophosphate may be introduced into a pyrophosphoric acid solution orinto a solution of an alkali metal pyrophosphate wherein thepyrophosphate ion is in considerable excess with respect to the combinedamount of plutonium and thorium. Following the dissolution of thecarrier precipitate, the plutonium is oxidized to the hexavalent state.This may be done by treatment of the solution with a suitable oxidizingagent as determined by the oxidation potential of plutonium given aboveand reference to a standard oxidation-reduction table. Particularlysuitable oxidizing agents have been found to be the alkali metalbismuthates, alkali metal permanganates, and alkali metal dichromates.Following the oxidation of the plutonium to the hexavalent state, thecarrier precipitate may again be precipitated by introducing a solublecompound containing the carrier cation into the solution so that theratio of carrier cation to pyrophosphate ion approaches stoichiometricproportions. The precipitate is then separated from the solution. Thisstep not only frees the plutonium from the carrier but is also effectivein removing certain additional contaminants from the solution. Thus,those fission products, such as zirconium and tin, having a valencestate of +4, but no higher valence state, which may have been carried bythe original carrier precipitate, will again be carried by the carrierprecipitate, since the oxidation step will not change these cations fromthe pyrophosphate insoluble state. Even fission products having bothtetravalent and higher oxidation states may be separated from plutoniumby this process by the suitable selection of an oxidizing agent in thisfinal step so that the plutonium but not the contaminant is oxidizedfrom the tetravalent state.

The decontamination of the plutonium may be greatly increased bycarrying out the process as a cycle and repeating the cycle. Thus,following the re-precipitation of the carrier precipitate, the plutoniummay be reduced to the tetravalent state, thus completing the cycle andthe cycle repeated as often as required to obtain the decontaminationdesired.

. This invention will be further illustrated by the following specificexample.

Example Four hundred fifty micrograms of Th(NO and 600 micrograms ofLaCl were dissolved in l M HCl. Tracer quantities of Pu(NO and Pu(NOwere introduced into this solution. Two hundred sixty-six micrograms ofNa P O were then introduced into an aliquot of the above solution havinga final volume of 0.12 ml. A white crystalline precipitate formed andthis was digested at room temperature with stirring for eight minutes.This precipitate was then separated from the solution by centrifugation.The solution and the precipitate were analyzed by radiometric methodsand it was found that 98.4% of the tetravalent plutonium had carriedwith the ThP O More than 99% of the trivalent plutonium and more than99% of the lanthanum remained in the solution. The ThP O --PuP O wasthen dissolved in a Na P O solution. Following the separation of the ThPO plutonium carrier precipitate from the solution containing thelanthanum and trivalent plutonium, thestate by treatment with NaBiO Thesolution is then contacted with 3000 micrograms of Th+ (as thenitrate),-

thus causing a precipitate of ThP O- to form. This precipitate wasdigested for ten minutes at room temperature with stirring and thenseparated from the solution.

Analysis of precipitate discloses that less than 1% of the plutoniumcarries with the precipitate.

It is to be understood, of course, that the above example is merelyillustrative and does not limit the scope of this invention. Othermixtures of plutonium with elements of lower atomic numbers may beemployed as starting materials and other acids and other cations' may besubstituted for those of the above example, with-' m the scope of theforegoing description. Likewise, the process of the example may bemodified by in corporating other purification steps in addition to theuse of the steps previously described. In general, it may be said thatthe use of any equivalents or modifications of procedure which wouldnaturally occur to one skilled,

in the art is included in the scope of this invention. Only suchlimitations should be imposed on the'scope of this invention as areindicated in the appended claims.

5 What is claimed is: 1. A method of purifying plutonium, whichcomprises introducing into an aqueous acidic solution of tetravalentplutonium a soluble thorium compound and a soluble pyrophosphatecompound, separating the precipitate thus formed from the solution,dissolving said precipitate, adding to the solutionthus formed anoxidizing agent having a potential more negative than 1.11 volts,whereby the tetravalent plutonium is converted to the hexavalent state,precipitating thorium pyrophos-.

phate in the resulting solution, and separating said thoriumpyrophosphate from the solution of oxidized plutonium.

2. A method of separating plutonium from elements with which it isnormally contaminated in neutron irradiated uranium, which comprisesforming an aqueous acidic solution containing said plutonium in thetetravalent state and contaminants, introducing into said solu-References Cited in the file of this patent Carney et al.: J. Am. Chem.Soc. vol. 26, pages 1134-1143 (1914).

King: AEC Document CN2726, Dec. 1, 1944, pages 6-8. (Copy available onmicrocard from ABC.)

Cunningham: AEC Document N-2205, page 1, Table 1, pages 32 and 42, Jan.16, 1946. Declassified Nov. 22, 1957, which reports work done by Miller,CN-979, page 29, Sept. 30, 1943, the Miller date being relied upon.

1. A METHOD OF PURIFYING PLUTONIUM, WHICH COMPRISES INTRODUCING INTO ANAQUEOUS ACIDIC SOLUTION OF TETRAVALENT PLUTONIUM A SOLUBLE THORIUMCOMPOUND AND A SOLUBLE PYROPHOSPHATE COMPOUND, SEPARATING THE